1. Field of This Invention
This invention relates to a process for the prevention and/or retardation of the putrefaction of skins, hides and pelts.
2. Prior Art
Leather is made from hide in three steps. First, there is removal of undesirable constituents such as hair, flesh, fat and some interfibrillary matters, leaving a concentrated network of high-protein collagen fibers, greatly softened and interspaced with water. Tanning (i.e., treating the hide with an agent, called tannin) follows which displaces the water and then combines with and coats the collagen fibers. Tanning increases resistance to heat, hydrolysis (decomposition caused by water), and microorganisms. Then there is finishing to obtain proper thickness, moisture, lubrication, and aesthetic appeal. So leather is esentially animal skin protein combined with tannins, small amounts of oils, dyes, finishes, and moisture.
A serious problem in the leather industry is that the rawhides or skins rapidly putrefy, causing serious economic loss and limits the time between skinning and tanning. A means is needed to prevent or retard the rawhides or skins so that the economic loses due to putrefaction between skinning and tanning are lessened.
Early on skins were scrapped, and then sun dried (which prevented rapid decay) but the skins became hard. Animal fats were rubbed into the dried hides to make them soft and pliable. It was found that hair, flesh, fat and the like should be removed. Wood ash and lime have been used to remove hair. Acid deliming and bating with enzymes have been used.
Fish oils have been rubbed into animal skins to make the furs soft and durable.
During preparation and shipping of hides for tanning, protection against excessive heat, humidity, rain and pests is essential. Raw hides are cured or preserved to prevent decay that begins within hours of slaughter. Curing consists of dehydration (of which there are many methods, some relatively costly) without disturbing skin structure.
The purpose of pretanning or beamhouse, operations is to remove undesirable constitutents and to condition the skin for tanning. The first operation in the tannery beamhouse is the soaking of cured hides in water to rehydrate the hide to its original flaccid condition and to remove dirt, salt, and some soluble proteins.
Dehairing is then effected. Liming, in which soaked hides, are treated with lime for one to two weeks to dehair, is still an old standard practice. By addition of such agents as sodium sulfide, dehairing time is reduced to a few hours. For dehairing sheepskins, lime sulfide paste is applied on the flesh side to save the wool, which is pulled in 3 to 12 hours. Liming not only removes hair, fats, and soluble proteins but also swells and conditions fiber structure.
A hide with its hair and flesh removed is known as a pelt (but not to be confused with fur pelts). Deliming is done partly or fully to remove excess alkali and is accomplished by treating with such mild acids as boric, lactic or such acid salts as ammonium chloride or sulfate and sodium bisulfite. The material is then bated to remove interfibrillary proteins and produce a clean white pelt. Bating is essential for special types of hides, such as those for gloving and glace kid leathers. Delimed, bated stock is pickled with a mixture of sulfuric acid and salt in a paddle or drum to prepare it for either long storage or mineral tannage. Fatty skins are degreased with detergent and paraffin solvents, washed and repickled.
There are a number of types of tanning methods. One is oil tanning, which can be combined with other types of tanning. One type of oil tanning produces chamois leather. The special properties of chamois leather result from its open structure. Chamois leather is tanned with aldehydes and peroxides, and coated with polymers from oxidation of fish oil used in the tanning process. Long liming, bating, and mechanical pressing open up and split in fibres; stocking and pounding with fish oil and hanging in heated rooms oxidizes the fish oil. Compounds of copper, manganese, and cobalt accelerate oxidation. Excess oil is washed off with washing soda solution or wetting agents. Though cod oil is favored, sardine, rape and safflower oils are also used. Sulfochlorinate hydrocarbons has been substituted for fish oil when producing white chamois leather. Japanese white leather and Ethiopian red leather are still produced with rape and safflower oils.
Smoking of hides and skins involves a reaction with aldehydes, several of which are used for tanning. Oxidation of oil also produces aldehydes. American Indians still make leather by a process that combines oil and smoke curing.
Bated pelts are tanned with formaldehyde solution for two to four hours and the excess flesh and bacterial matter in the hide is removed by treatment with magnesium acetate or hydrogen peroxide to produce white leathers.
Tawing is another method. Oils or greases are rubbed or worked into the leather. Currying is the tawing of leathers which have been tanned first by other methods.
After tanning, leather is usually dyed. Unless lubricated, leather dries hard. Dyed leathers are treated with oils and fats for lubrication softenss, strength, and waterproofing. Oils and fats are incorporated individually, or in blends, in natural condition, as emulsion, or in solvents, by hand or in a drum.
Vegetable tanned light leathers are oiled by swabbing groundnut (peanut) oil on the grain surface. Sole leather is drummed with a mixture of vegetable oil, mineral oil, and small amounts of sulfated oil, epsom salts, and glucose. The oil keeps loose tanning material from rising to the grain during drying, and produces supple, light-colored leather. Belting leather is treated by hand or in a heated drum with a mixture of cod oil, tallow, wool grease, stearine, and paraffin wax. Currying is similar, incorporating such mixtures as a hot melt.
Light leathers are fat-liquored in a drum with an oil and water emulsion. The uniform penetration of the leather by fat liquor gives a soft, stretchy, loose-grained leather; if the fat liquor is deposited only on the surface, the leather is resilient and tight. Raw oils mixed with emulsifiers give desired properties of softness and pliability to suede. Raw oils mixed with soaps or sulfated oils are commonly used. Such vegetable oils as castor, pam, and groundnut; animal oils as tallow and neat's-foot; marine oils as cod, sperm, and sardine; and mineral oils and fatty alcohols are sulfated with sulfuric acid at low temperatures. The greater the reaction, the greater the penetration and stability of the product. Sulfated oils are favoured for gloving, suede, and soft leathers. Oil may be intorduced into leather along with a solvent and the solvent then evaporated.
Fatty acids and waxes are used for shower proofing or waterproofing of leathers. To avoid the yellowing of aging, especially of white leathers, synthetic oils and sulfated coconut oils are used.
After dying and fat-liquoring, leather contains 45 to 60 percent water and is dried to about 14 percent moisture, chemical and physical reactions taking place. When leather dries to a paler shade, loose tannins, dyes, and oils spread uniformly, penetrate deeply, and are fixed firmly. Uneven drying causes the migration of unfixed tannin dye and oil to the surface.
Mammalian hides and skins are divided into three layers distinct in structure and origin. These are (1) a thin outer layer of epithelial cells called the epidermis; (a thick layer called corium, or dermis; and (3) subcutaneous adipose or flesh layer. In tanning, the epidermis and flesh layers are removed; the corium is tanned into leather. One layer of the corium containing the grain membrane and hyaline layer, together with the arrangement of hair pores, gives a distinct grain surface pattern for each species of animal. The other corium layer is composed of large collagen fibre bundles interwoven at an angle in a three-dimensional network.
Fur is the fine, soft, hairy covering (or coat) of a mammal. Fur usually consists of a layer of relatively short, soft, barbed hairs next to the skin, helping to maintain body temperature, and a top layer formed by longer, stiff, smooth hairs growing up through the underlying layer, serving to shed rain. Various processes are used that make animal furs wearable and enhance their attractiveness. Fur pelts are animal skins with the hair forming the body covering remaining intact. The true furs consist of a soft, dense, undercoat, called ground hair, underhair, or underwool, and a longer protective covering called guard hair or top hair. The pelts of certain animals, lacking either guard or ground hair, are not true furs, although used commercially as furs. Persian lamb, for example, sold as a fur, has only underwool and no guard hairs; monkey fur has guard hairs but no underhair. Mink, with its dense ground hair and long, glossy guard hair, is an example of a true fur.
The skin is composed of a lower layer, or dermis, consisting mainly of connective tissue, toughened during processing to form the leather; and the upper layer, or epidermis, composed mainly of nonliving cells. The epidermis is removing during processing.
Fur skins are dressed to make them suitable for use. The fur dresser aims at the creation of a soft, pliable leather; the removal of superfluous matter from the pelt; and the preservation and enhancement of the natural lusture of the fur. The details of the process vary with the nature and condition of the skin treated, but there are usually at least four distinct stages in the operation. First, there is the preliminary cleaning and softening of the pelt. Then fleshing (removal of fleshy matter from the skin) and stretching is achieved. (To preserve natural oils, furs are greased and rubbed soft, after which the grease is removed in soapsuds and the skins are dried.) Leathering, a tanning process is done that results in the formation of a leather on the skin. Then there is a final cleaning. Separate process in the fur dresser's art are unhairing, or plucking (the removal of guard hairs where necessary); shaving of the leather side to decrease weight, impart suppleness, and improve draping qualities; and sometimes shearing of the ground hair of such furs as fur seal, muskrat, racoon, and beaver to achieve a desired depth.
After the furs are dressed, they may then be dyed.